In my U.S. Pat. No. 3,532,679, issued Oct. 6, 1970, and entitled Hydrogels from Cross-linked Polymers of N-Vinyl Lactams and Alkyl Acrylates, I have described certain neutral hydrogels obtained by simultaneous polymerization and cross-linking of a mixture of an N-vinyl lactam, and alkyl acrylates. In my copending application Ser. No. 385,275 filed July 27, 1973, now U.S. Pat. No. 3,878,175 issued Apr. 15, 1975, I have described an improvement on my said patent, wherein a solution of the monomers in a hydrophobic solvent is simultaneously polymerized and cross-linked; whereby a highly absorbent, spongy, polymeric, neutral hydrogel is obtained.
I have now found that such N-vinyl lactam or other heterocyclic N-vinyl monomer based hydrogels can be modified, by incorporating an anionic monomer in the mixture of monomers being simultaneously polymerized and cross-linked, so that a hydrogel having anionic functionality, and thus new and useful properties, is obtained.
A number of synthetic polymeric materials, which contain acidic groups which impart anionic functionality thereto are known in the art; possibly the most widely available and best known of such anionic synthetic resins, are the cation exchange resins available under such trade-names as Amberlite, Dowex, Permutit and Zeocarb. In genereal the so-called "weak" cation exchange resins contain carboxylic groups while the so-called "strong" cation exchange resins contain sulfonic groups. However, such cation exchange resins are not hydrogels.
As disclosed in my prior U.S. Pat. No. 3,532,679, supra, various cross-linked hydrogels are known in the art. However, practically all of these known hydrogels are neutral hydrogels and are not inoic in character. While in U.S. Pat. No. 3,689,634, issued Sept. 5, 1972 to Kliment, Vacik, Majkus and Wichterle, entitled Protracted Activity Oral Hydrogel Bead; there is a broad suggestion that "it is also possible to replace the non-ionizable cross-linked hydrogels by physically similar hydrogels containing also ionizable groups"; the only examples of ionic hydrogels disclosed in this patent are: "A porous hydrogel capable of exchanging cations prepared by copolymerizing a mixture of 35 parts of methacrylic acid, and 30 parts of a 25 percent aqueous solution of maleic anhydride," disclosed in Example 8 at the top of column 8 of the patent; and "A copolymer prepared from 97 parts of ethylene glycol monomethacrylate, 2 parts of methacrylic acid and 1 percent of ethylene glycol bis-methacrylate by suspension polymerization in a concentrated, aqueous solution of sodium chloride, using 0.05 parts of diisopropyl percarbonate as a polymerization initiator," disclosed in Example 9 at the middle of column 8 of the patent. These prior art ionic hydrogels are obviously substantially different from those of the present invention; inter alia, the prior art hydrogels contain no N-vinyl lactam, or other N-vinyl heterocyclic monomer units and thus are structurally different from those of the present invention, and would be lacking in properties attributable to such N-vinyl lactam etc. units.
Other ionic synthetic polymeric materials which are known in the prior art, are the self-stabilizing polymer latices obtained by emulsion polymerization techniques in which a copolymerizable surfactant is used as an emulsifier in the preparation of the aqueous emulsion of monomer(s) to be polymerized. In the course of the polymerization, these copolymerizable surfactants copolymerized with the monomer or mixture of other monomers being polymerized and become an integral part of the resulting polymer so that the polymeric material so obtained contains ionic (acidic) groups. As examples of acidic, ionic, copolymerizable surfactants which have been so used may be mentioned the polymerizable .alpha.-methylene carboxylic acid esters (e.g., the acrylic and methacrylic acid esters) of hydroxyalkane sulfonic acids such as those disclosed in U.S. Pat. No. 3,024,211 and 3,033,833 both to Le Fevre and Sheetz and U.S. Pat. No. 3,617,368 to Gibbs and Wessling; also the sulfate esters of hydroxyalkyl acrylates and methacrylates disclosed in my U.S. Pat. No. 3,839,393 issued Oct. 1, 1974; also the phosphate esters of hydroxyalkyl acrylates and methacrylates disclosed in my copending application Ser. No. 321,229, filed Jan. 5, 1973, now U.S. Pat. No. 3,855,364; and the sulfates of polymerizable ethylenically unsaturated alcohols and their alkylene oxide adducts disclosed in my application Ser. No. 321,228, filed Jan. 5, 1973, now U.S. Pat. No. 3,875,202. Such copolymerizable surfactants are also used to impart hydrophilic properties to the resulting polymer, to improve the receptivity of the resulting polymer to basic dyes and other purposes more fully described in the above patents; however, none of the polymers heretofore produced by their use have, to the best of my knowledge, been in the form of hydrogels.
One of the outstanding advantages of the hydrogels of the present invention which contain anionic groups, as compared with the non-hydrogel form of anionic polymeric materials heretofore obtained by the use of anionic copolymerizable monomers, such as those mentioned above which contain carboxylic, sulfonic or sulfate groups, is that the hydrogel form of the anionic polymers of the present invention permits and assures much more intimate contact between the anionic groups of the anionic polymeric hydrogel and any basic material which it is desired to combine or complex therewith. In the presence of water the anionic hydrogels of the present invention are quite permeable and swollen. Due to this swelling the water, and any basic material dissolved or dispersed therein, of an aqueous medium with which these anionic hydrogels are used, or come in contact with during use, can readily diffuse or be transported throughout the hydrogel. As a result, combination or complexing of basic materials with the anionic groups of the polymeric hydrogel can and does take place throughout the hydrogel in contrast for example, with the essentially surface action in the case of cation exchange resins. This swelling also increases the distance between the anionic groups of the hydrogel and this is also conductive to more complete reaction. Thus basic materials can be combined or complexed much more efficiently and completely with the anionic groups of the anionic polymeric hydrogels of the present invention; and, conversely, basic materials which are complexed or otherwise combined with the anionic groups of the anionic polymeric hydrogels of this invention may be more efficiently released therefrom and transferred to an aqueous medium with which they are used; especially in comparison with corresponding ion exchange resins.